The present invention relates to a wound magnetic core constituted by a thin ribbon of a fine crystalline, soft magnetic Fe-base alloy and a method of producing it, and more particularly to a wound magnetic core constituted by a thin ribbon of a fine crystalline, soft magnetic Fe-base alloy coated with a heat-resistant insulating layer, thereby showing excellent high-frequency magnetic properties, high-voltage magnetic properties, etc. and a method of producing it.
There have recently been developed as magnetic materials having excellent high-frequency properties, fine crystalline, soft magnetic Fe-base alloys having extremely fine crystalline structures having an average grain size of 1000 .ANG. or less (EP0271657 and Japanese Patent Laid-Open No. 63-302504).
These fine crystalline, soft magnetic Fe-base alloys include a fine crystalline, soft magnetic Fe-base alloy having the composition represented by the general formula: EQU (Fe.sub.1-a M.sub.a).sub.100-x-y-z-.alpha. Cu.sub.x Si.sub.y B.sub.z M'.sub..alpha.
wherein M is Co and/or Ni, M' is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, and a, x, y, z and .alpha. respectively satisfy 0.ltoreq.a.ltoreq.0.5, 0.1.ltoreq.x.ltoreq.3, 0.ltoreq.y.ltoreq.30, 0.ltoreq.z.ltoreq.25, 5.ltoreq.y+z.ltoreq.30 and 0.1.ltoreq..alpha..ltoreq.30, at least 50% of the alloy structure being occupied by fine crystal grains having an average grain size of 1000 .ANG. or less; and a fine crystalline, soft magnetic Fe-base alloy having the composition represented by the general formula: EQU (Fe.sub.1-a M.sub.a).sub.100-x-y-z-.alpha.-.beta.-.gamma. Cu.sub.x Si.sub.y B.sub.z M'.sub..alpha. M".sub..beta. X.sub..gamma.
wherein M is Co and/or Ni, M' is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, M" is at least one element selected from the group consisting of V, Cr, Mn, Al, elements in the platinum group, Sc, Y, rare earth elements, Au, Zn, Sn and Re, X is at least one element selected from the group consisting of C, Ge, P, Ga, Sb, In, Be and As, and a, x, y, z, .alpha., .beta. and .gamma. respectively satisfy 0.ltoreq.a.ltoreq.0.5, 0.1.ltoreq.x.ltoreq.3, 0.ltoreq.y.ltoreq.30, 0.ltoreq.z.ltoreq.25, 5.ltoreq.y+z.ltoreq.30, 0.1.ltoreq..alpha..ltoreq.30, .beta..ltoreq.10 and .gamma..ltoreq.10, at least 50% of the alloy structure being occupied by fine crystal grains having an average grain size of 1000 .ANG. or less.
These alloys can usually be obtained by preparing amorphous alloys and then subjecting them to a heat treatment at a temperature higher than their crystallization temperatures.
When thin ribbons of the above alloys are used to produce wound magnetic cores for saturable reactors, transformers, etc., they are preferably insulated by insulating tapes such as polyimide films, polyethylene terephthalate films or insulating layers of oxide powders such as SiO.sub.2, MgO, Al.sub.2 O.sub.3, etc. to decrease eddy current losses which are main causes of core losses of the wound magnetic cores (Japanese Patent Laid-Open No. 63-302504).
It was also proposed as alternative methods for achieving the inter-laminar insulation of wound magnetic cores that organometallic compounds such as metal alkoxides are coated to increase heat resistance temperatures of the insulating layers (Japanese Patent Laid-Open No. 63-110607), and that a mixture of a sol of partially hydrolyzed SiO.sub.2 --TiO.sub.2 metal alkoxide and various ceramic powders is coated (Japanese Patent Laid-Open No. 63-302504).
However, in the case of the above fine crystalline, soft magnetic Fe-base alloys having extremely fine crystalline structures having an average grain size of 1000 .ANG. or less (determined from maximum diameters of grains), their heat treatment temperatures are as high as 500.degree. C. or even higher to cause crystallization, and the alloys become somewhat brittle after the heat treatment. Accordingly, the heat treatment should be conducted after the thin ribbons are coated with insulating layers. Therefore, insulating materials showing excellent heat resistance are needed.
However, in the case of insulating films, even though polyimide insulating films showing relatively high heat resistance are used as insulating materials, they are deteriorated at heat treatment temperatures of 500.degree. C. or higher, failing to maintain sufficient insulation.
Alternatively, when ceramic powders such as SiO.sub.2, MgO, Al.sub.2 O.sub.3, etc. are used as insulating materials, since the ceramic particles are not completely bonded to the thin alloy ribbons, the insulating layers tend to be flowed away when the wound magnetic cores are immersed in a flowing cooling fluid.
In addition, since voltage of several tens of kV or more is applied to wound magnetic cores for transformers and saturable reactors for supplying high-voltage pulses as disclosed in Japanese Patent Laid-Open No. 63-229786, the conventional insulating layers inevitably suffer from increase in core losses due to insufficient insulation.
Insulating materials of metal alkoxides in which fine ceramic particles are dispersed are considered promising because of their heat resistance. However, in the case of the insulating layer made of a sol of partially hydrolyzed SiO.sub.2 --TiO.sub.2 metal alkoxide and fine ceramic particles disclosed in Japanese Patent Laid-Open No. 63-302504, such metal alkoxide (partially hydrolyzed sol) shows heat shrinkage ratio (mainly due to cross-linking reaction), which is extremely different from the shrinkage ratio (due to fine crystallization) of the fine crystalline, soft magnetic Fe-base alloy. Accordingly, the resulting insulating layer has a large residual internal stress, which leads to the deterioration of magnetic properties of wound magnetic cores constituted by thin ribbons of the fine crystalline, soft magnetic Fe-base alloys.